1. Field of Invention
The present invention relates to a piezoelectric resonator such as a quartz crystal resonator, and more particularly, to a supporting structure of a piezoelectric resonator and a method for manufacturing the same.
2. Description of Related Art
A piezoelectric resonator unit such as a quartz crystal resonator unit comprising a piezoelectric body such as a quartz crystal element having an electrode formed on the surface thereof is popularly used in an oscillating circuit for generating a prescribed frequency. Recently, there is an increasing demand for a piezoelectric resonator unit giving stable characteristics at a high accuracy as an oscillation source of a reference signal for communication equipment. FIG. 11 illustrates a front view (FIG. 11(a)) and a side view (FIG. 11(b)) of a schematic configuration of a typical conventional piezoelectric resonator unit. A piezoelectric resonator 10 of the piezoelectric resonator unit 8 is provided with a resonator element 5 comprising a thin and flat quartz crystal piece 1 as a piezoelectric body, having an electrode 3 formed on each of the two surfaces thereof by vapor deposition or the like, and a cylindrical plug 11 supporting the resonator element 5. The plug 11 has a metal frame 13 surrounding an insulating member 12 made of a material such as covar glass having an electrolytically plated outer periphery, and two round bar-shaped leads 15 running through this insulating member 12. Leading ends 16 of these two leads 15 are respectively solder-connected to connecting electrodes 4 of the resonator element 5 so as to be conductive with the exterior of the plug 11 serving as a supporting member via the leads 15. The leads 15 serve also to mount the resonator element 5 onto the plug 11. Further, the piezoelectric resonator 10 shown in FIG. 11 mounts the resonator element 5 by holding it between the leads 15 to form a supporting structure with a very high rigidity.
Main steps for assembling a piezoelectric resonator unit by the use of such a piezoelectric resonator 10 are illustrated in FIG. 12. After the completion of Step 21 of mounting the resonator 5 onto the plug 11 serving as the supporting member as described above, a final resonance frequency adjustment is carried out in Step 22 by adjusting the film thickness of the electrode 3 of the resonator element 5 through vapor deposition or sputtering. Then, in Step 23, the piezoelectric resonator 10 is inserted and sealed into a case 9 in vacuum or in an inert gas atmosphere to assemble the piezoelectric resonator unit 8. Then, after conducting tests of frequency, CI-value and temperature characteristics in Step 24, the product is shipped.
As described above, in addition to the demand for a piezoelectric resonator or a piezoelectric oscillator comprising a combination of a piezoelectric resonator unit and a semiconductor as a clock source of a CPU and the like, a demand as also recently increasing for a reference signal source of communication equipment, and it is becoming increasingly necessary to have a high accuracy and stable characteristics more than the conventional reference signal source of communication equipment. Particularly, room-temperature aging property is required to be improved from about xc2x13 to 5 ppm/year to about xc2x11 ppm/year, and further, there is a demand for a piezoelectric resonator unit or a piezoelectric oscillator, in consideration of the application in a portable equipment, excellent in impact resistance and high in durability.
In the conventional piezoelectric resonator unit shown in FIG. 11, in which the leads are connected to the electrode by the use of solder 17, the connecting step exhibits a satisfactory operability, and a high connecting strength is available. When left at a high temperature of 80 to 125xc2x0 C., however, solder may diffuse into the electrode, this resulting in easier deterioration of aging property and in easier occurrence of fluctuation of frequency. In addition, mounting by the use of soldering requires a preheating step at about 150 to 250xc2x0 C., although for a short period of time, and a heating step at about 350xc2x0 C. As a result, in a certain state of heating, there may be a distortion of temperature characteristics. When the resonator element is AT-cut, deviation occurs from an ideal cubic curve, and this makes it difficult to obtain a highly accurate frequency even by performing temperature correction. Such a phenomenon is believed to be attributable to the fact that local heating of the piezoelectric body upon connection causes a difference in temperature, crystals having different properties such as xcex2-quartz crystal having no piezoelectric phenomenon are produced in the piezoelectric body, though to a minimum extent.
Furthermore, the conventional piezoelectric resonator unit shown in FIG. 11, in which the resonator element is connected to the plug in a state with a very high rigidity, is highly resistant to micro-vibration. The unit however has a poor resistant to a strong impact such as dropping, thus resulting in breakage or peeling of the resonator element, and hence making the unit unserviceable at a high probability.
To overcome these defects, Japanese Unexamined Patent Publication No. 6-303,077 discloses a technique of connecting the leads and the resonator element by means of a conductive adhesive in place of solder, and connecting the leads only to a side surface of the resonator element. Although there is not an explicit description in the Japanese Unexamined Patent Publication No. 6-303,077, with the use of such a conductive adhesive, solder diffusion can be avoided even when left exposed to a high temperature as described above, thus permitting improvement of aging property, and absence of heating to a high temperature upon mounting leads to a slight distortion of temperature characteristics, thus making it possible to provide a product of a very high accuracy. In this technique, flattened leads are attached to one side surface of the resonator element, in place of mounting the resonator element by holding it between high-rigidity bar-shaped leads. It is therefore known that a high impact resistance is available, and the probability of becoming unserviceable as a result of dropping (dropping property) becomes lower. Even in a resonator element adopting such a supporting method, a piezoelectric resonator unit 8 using a piezoelectric body 2 of a convex shape as shown in FIG. 13 results in a relatively large weight of the resonator element 5, and stress tends to be concentrated at the constriction at the leading end of the mounting section, thus causing an occurrence of peeling. It is therefore impossible to achieve a remarkable improvement in the dropping property.
Further, in a piezoelectric resonator 10 shown in FIG. 13, the leading end 16 of the bar-shaped lead 15 is flattened into substantially a U shape, and the leading end 16 and a connecting electrode 4 are connected with a conductive adhesive 19. A plug-side edge 5a of the resonator element 5 is mounted onto the plug 11 whit a non-conductive adhesive 18. There is therefore available an improved operability as compared with the conventional piezoelectric resonator 10 shown in FIG. 11, in which mounting is accomplished so that both sides of the resonator element are in contact with the bar-shaped leads. However, in view of the time and labor required for injecting the conductive adhesive into an appropriate gap formed between the leading end 16 and the connecting electrode 4, and for holding the resonator element 5 and the plug 11 at prescribed positions with the use of devices and jigs until setting of the injected conductive adhesive, operability of the step of causing the coated adhesive to set is not high as compared with the step for carrying cut soldering, and the devices and jigs for positioning the resonator element and the plug have a low operating efficiency.
An object of the present invention is therefore to provide a piezoelectric resonator and a piezoelectric resonator unit, in which a resonator element is mounted on a supporting member by means of a resin adhesive, excellent in aging property and high in accuracy, wherein impact resistance is further improved. Another object of the invention is to provide a piezoelectric resonator and a method for manufacturing the same, which permits the efficient manufacture of a piezoelectric resonator and a piezoelectric resonator unit in which mounting is accomplished by the use of such a resin. A further object of the invention is to provide at a low cost a piezoelectric resonator and a piezoelectric resonator unit for a communication equipment of a high demand by providing a piezoelectric resonator and a piezoelectric resonator unit high in productivity and accuracy.
To achieve the foregoing objects of the invention, in contrast to the conventional piezoelectric resonator in which a piezoelectric resonator element is fixed directly to a supporting member with an adhesive, or fixed by soldering using a metal requiring heating for melting, resulting in a rigid structure, the present invention permits improvement of impact resistance by achieving a soft structure through connection of the piezoelectric resonator element to the supporting member only through leads by the use of a conductive resin. More specifically, in the piezoelectric resonator of the invention comprising a piezoelectric resonator element comprising a piezoelectric body having an electrode formed on the surface thereof, a supporting member supporting the piezoelectric resonator element, and a plurality of leads mechanically connecting the piezoelectric resonator element to the supporting member and permitting electrical connection thereof, each of the leads is provided with a flat leading end portion correctable substantially in parallel with the electrode, and opening substantially in a U shape toward the leading end. A connecting layer is formed with a conductive resin in the gap between the leading end portions and the electrode, and further, the piezoelectric resonator element is supported by the leads so as to form a gap between the supporting member and the piezoelectric resonator element.
In the piezoelectric resonator of the invention, for example, the piezoelectric element is attached to the leads, so that the edge of the piezoelectric resonator element on the side facing the supporting member is substantially positioned at the end on the side of the opening of the leading end portion substantially in a U shape facing the supporting member. As a result, the piezoelectric resonator element is supported by the leads having ends shaped into a sheet by means of an elastic resin in a state in which the piezoelectric resonator element is floating from the supporting member, thus achieving a soft (spring) structure supporting mechanism. Consequently, the supporting mechanism of the soft structure by the use of highly elastic leads can absorb a strong impact such as that occurring in being dropped. Occurrence of an inconvenience such as peeling even upon application of a strong impact can therefore be prevented, and it is possible to provide a piezoelectric resonator unit high in impact resistance. As a result, even in a piezoelectric resonator unit using a resonator element poor in impact resistance resulting from a relatively large weight and tending to cause stress concentration at the connecting portion, for example, in a convex one, it is possible to improve the dropping property and increase durability.
Further, by mounting the piezoelectric resonator element so that the edge thereof substantially matches the end of the U-shaped opening of the leading end, the attaching position can be easily confirmed, thus minimizing variation of the position. It is therefore possible to avoid variation of impact resistance and flowing of the conductive resin through the opening bottom at the leading end portion opening in a U shape, which may cause a short circuit.
By shaping the leading end of the round bar-shaped lead into a sheet (flat) shape, the leading end portion cross-section is tapered, converging toward the leading end. As a result, it is possible to ensure a sufficient thickness of a conductive resin (conductive adhesive) such as a silver paste in a sufficient quantity between the resonator element and the leads only by coating the conductive resin onto the U-shaped leading end portions of the leads. This permits connection of the resonator element and the leads with a sufficiently high strength by the use of the conductive resin, and additionally, the resistance at the contact portion can remain at a low level. Further, because the leads and the resonator element can be connected with the conductive resin, the piezoelectric resonator element is never exposed to high temperatures as compared with the conventional piezoelectric resonator using soldering. It is therefore possible to provide a piezoelectric resonator unit satisfactory in aging property, high in accuracy and excellent in temperature characteristics. According to the present invention, as described above, it is possible to provide a piezoelectric resonator with a further improved impact resistance (dropping property), high in durability and accuracy, and having stable properties, and by sealing this piezoelectric resonator in a protector such as a case, it is possible to provide a piezoelectric resonator unit of an excellent quality.
Prior to forming a connecting layer, a temporary fixing layer can be formed by coating a UV-setting resin having a very short setting time so as to temporarily fix the leading end portions and the piezoelectric resonator element resonator element). Provision of this temporary fixing layer permits positioning of the supporting member and the piezoelectric resonator element prior to coating the conductive resin. Thereafter, therefore, the conductive resin an be injected into the gap between the leading end portions and the electrode without the need to hold the supporting member and the piezoelectric resonator element at a prescribed position by means of devices and jigs to wait for the subsequent setting, thus permitting considerable improvement of operability upon mounting the resonator element onto the leads. It is also possible to improve reliability of the connecting portion between the leads and the electrode since there is no possibility of an occurrence of a positional shift during operation. Further, by providing the temporary fixing layer on the side facing the adjacent lead, it is possible to impart the function of preventing short circuit between leads upon coating the conductive resin.
The conductive resin may be previously coated onto at least any one of the leading end portions and the electrode and then they may be stuck together. By previously coating the conductive resin, the gap between the leading end portions and the electrode can be filled with the conductive resin in a sufficient quantity, so that a good contact and a high adhesion are available. It is therefore possible to conduct temporary fixing by only holding for a short period of time without the need to use a temporary fixing layer, and by leaving as it is without the use of jigs, a sufficient adhesion is available. As a result, it is possible to omit the UV-setting type resin for temporary fixing. As compared with soldering, the UV-setting resin is excellent in high-temperature stability. However, because a slight amount of out-gas is observed at high temperatures, omission of the UV-setting type resin permits further improvement of high-temperature properties.
By forming a reinforcing layer by coating the conductive resin or the non-conductive resin so as to cover at least the connecting layer and the leading end portions, it is possible to improve the connecting strength between the leads and the piezoelectric resonator element, and reliability of the connecting portion through prevention of the occurrence of peeling. Further, because notch portions formed by the leads and the piezoelectric resonator element are eliminated, stress concentration does not occur, and impact resistance including dropping property can be improved.
In the method for manufacturing the piezoelectric resonator of the invention, when attaching the piezoelectric resonator element comprising a piezoelectric body having an electrode formed on the surface thereof to the plurality of leads which mechanically connect to the supporting member and permit electrical connection thereof, i.e., when mounting the piezoelectric resonator element onto the supporting member such as a plug, there is provided a connecting step of forming a connecting layer of a conductive resin in the gap between the flat leading end portions connectable substantially in parallel with the electrode, opening substantially in a U shape toward the leading end and the electrode of the piezoelectric resonator element. In this connecting step, it is possible to use a process comprising a first step for forming a temporary fixing layer by coating a UV-setting type resin onto at least a part of the leading end portions of the leads and the piezoelectric resonator element, and a second step for forming a connecting layer by injecting a conductive resin at least into a gap between the leading end portions and the electrode.
By adopting the manufacturing method provided with such a temporary fixing step, it is possible to efficiently perform the mounting operation even by using a conductive resin requiring a longer period of time for setting than in soldering, without the need for occupying positioning devices and jigs for a long time, and to provide a highly reliable piezoelectric resonator at a low cost. By coating the UV-setting type resin onto the side facing the adjacent lead in the first step, it is possible to prevent the conductive resin from flowing onto the side of the adjacent lead, thus permitting further improvement of operability upon forming the connecting layer.
Prior to connecting the leading end portion to the electrode in the connecting step, a conductive resin may be coated onto the leading end portion or the electrode to form the connecting layer by stacking them together. In this method which permits omission of the UV-setting type resin, the connecting step can further be simplified. Since the connecting layer can be formed with the conductive resin in a sufficient quantity in the gap between the leading end portion and the electrode, a sufficient adhesion can be ensured, and resistance to the connecting portion can be reduced.
Further, by adopting a reinforcing step of forming a reinforcing layer by coating a conductive resin or a non-conductive resin so as to cover at least the connecting layer and the leading end portions of the leads, it is possible to improve strength and reliability of the connecting portion in mounting. The conductive resin injected in the foregoing second step should preferably have fluidity to some extent for ensuring a close contact between the leads and the electrode. In the reinforcing step, in contrast, a conductive resin or a non-conductive resin having a viscosity higher than that of the conductive resin coated in the second step should preferably be used for ensuring a sufficient strength and preventing diffusion. When adopting the method of previously coating the conductive resin onto the electrode or the leading end portions of the leads and then sticking them together, protection can be provided by applying a secondary coating of a conductive resin of the same type.